In modern houses and buildings, water sometime leaks through a roof, a basement wall or a floor. The leakages occur when the shingles, roof tar or tar-sand paper deteriorates or when the basement walls or floor crack. This may not be noticed right away until water leaks through the roof or extends to a visible region. It is advantageous to detect the leak early since the presence of water on the roof or floor results in deterioration of the roof or floor construction materials and may lead to accumulation of moisture and bacteria which are harmful to the human body. Even when the leak is discovered, it is still difficult to identify its actual location. For example, it is often require to remove a large area of the floor or wall decoration and insulation for a finished basement in order to identify the location of the leak. From the above comment, it is clear that there is a need to develop a method and a system to detect moisture or water in the early stage of the leak and to identify the location of the leak. In the present invention, a method is described which detects the electrical resistance between or current through two wires and indicates whether water leak is present. Ordinary rain water or city water contains ions of various elements. These ions migrate with the application of a voltage. Thus when a voltage is applied to water-covered or water-soaked construction materials, a current will be allowed to flow. This current will be used to sense and to identify the leak.